1. Field of the Invention
The present invention relates to an organic electroluminescent display (OELD) device, and more particularly, to a method and apparatus for driving an OELD device.
2. Discussion of the Related Art
In general, display devices include cathode-ray tubes (CRT) and various types of flat panel displays. However, the various types of flat panel displays, such as liquid crystal display (LCD) devices, plasma display panel (PDP) devices, field emission display (FED) devices, and electroluminescent display (ELD) devices, are currently being developed as substitutes for the CRT. For example, advantages of LCD devices include a thin profile and low power consumption. However, LCD devices require a backlight unit because they are non-luminescent display devices. Organic electroluminescent display (OELD) devices, however, are self-luminescent display devices. OELD devices operate at low voltages and have a thin profile. Further, the OELD devices have fast response time, high brightness, and wide viewing angles.
FIG. 1 is a circuit diagram illustrating an OELD device according to the related art. As illustrated in FIG. 1, the OELD device of the related art includes a gate line SL and a data line DL perpendicular to the gate line SL. A pixel includes a switching transistor T1, a driving transistor T2, a capacitor C, and an organic light emitting diode OLED. The switching transistor T1 is connected to the gate line SL and data line DL. A gate electrode of the driving transistor T2 is connected to the switching transistor T1. A source electrode of the driving transistor T2 is connected to a power line VDDL. A capacitor C is connected to the source and gate electrodes of the driving transistor T2. An anode of the organic light emitting diode OLED is connected to the driving transistor T2, and a cathode of the organic light emitting diode OLED is connected to a ground terminal VSS. A plurality of pixels having the above pixel structure are arranged in a matrix to form the OELD device.
When the switching transistor T1 is turned on, a data voltage is applied to the driving transistor T2 and a diode current (IOLED) is provided to the organic light emitting diode OLED to emit light. The capacitor C stores the data voltage applied to the driving transistor T2. The diode current (IOLED) is expressed as follows:IOLED=β/2(Vgs−Vth)2=β/2(VDDL−Vdata−Vth)2,where β is a constant; Vgs is a voltage between gate and source electrodes of the driving transistor T2; Vth is a threshold voltage of the driving transistor T2; Vdata is a data voltage; and VDDL is a power voltage. The diode current (IOLED) depends on a threshold voltage (Vth) of the driving transistor T2. Thus, the operation of a pixel is influenced by the threshold voltage (Vth) property of the driving transistor T2. The different pixels in the OELD device may have different threshold voltages (Vth) due to variations in fabrication processes. This threshold voltage variation causes the diode currents (IOLED) of different pixels to vary.
To resolve this problem, a voltage compensation type OELD device is suggested. FIG. 2A is a circuit diagram illustrating a voltage compensation type OELD device according to the related art. FIG. 2B is a waveform view illustrating signals applied to the OELD device of FIG. 2A.
As illustrated in FIG. 2A, a pixel includes four transistors T1, T2, T3, and T4. A switching transistor T1 is connected to a gate line SL and a data line DL. A driving transistor T2 is connected to a power line VDDL. An emitting control transistor T4 is connected to an organic light emitting diode OLED, and a gate electrode of the emitting control transistor T4 is connected to an emitting control line ECL. A sampling transistor T3 is connected to gate and drain electrodes of the driving transistor T2. A gate electrode of the driving transistor T3 is connected to a sampling line SPL. A first capacitor C1 is connected to a drain electrode of the switching transistor T1 and a source electrode of the driving transistor T2. A second capacitor C2 is connected to the drain electrode of the switching transistor T1 and the gate electrode of the driving transistor T2.
As shown in FIG. 2B, when the gate line SL is applied with a low level gate voltage, the switching transistor T1 is turned on, and thus the driving transistor T2 is turned on. When the sampling line SPL is applied with a low level sampling clock signal, the sampling transistor T3 is turned on. During a sampling time ST, an offset voltage of the driving transistor T2 is sampled, and the offset voltage is stored in the second capacitor C2. The gate electrode of the driving transistor T2 has a voltage (VDDL−Vth) during the sampling time ST. Then, when the sampling line SPL is applied with a high level sampling clock signal, a data voltage (Vdata) is applied to the data line DL and stored in the first capacitor C1 through the turned-on switching transistor T1. When the data voltage (Vdata) is applied, the gate electrode of the driving transistor T2 has a voltage (VDDL−Vth−Vdata).
A high level emitting control signal is applied to the emitting control line ECL during the sampling time ST to turn off the emitting control transistor T4. By turning off the emitting control transistor T4, a diode current (IOLED) does not flow through the organic light emitting diode OLED. After the sampling time ST, a low level emitting control signal is applied to the emitting control transistor T4, and the emitting control transistor T4 is turned on such that the diode current (IOLED) flows through the organic light emitting diode OLED.
As explained above, the threshold voltage (Vth) of the driving transistor T2 is sampled and stored before the data voltage (Vdata) is applied to operate the driving transistor T2. Accordingly, when the driving transistor T2 is normally operated to display an image, the threshold voltage (Vth) property of the driving transistor is offset. Hence, the diode current (IOLED) variation between the different pixels due to a threshold voltage (Vth) deviation of the driving transistor T2 is compensated, and the pixel operates without an influence of the threshold voltage (Vth) property.
In addition, an S-factor sometimes influences the operation of the driving transistor T2. That is, the diode current (IOLED) is influenced by not only the threshold voltage (Vth), but also by the S-factor. For instance, a high gray level (i.e., bright gray level) displayed by a high diode current (IOLED) is influenced by the threshold voltage (Vth) property. In other words, the high gray level is not influenced by the S-factor property of the driving transistor T2. On the other hand, a low gray level (i.e., dark gray level) displayed by a low diode current (IOLED) is influenced by the threshold voltage (Vth) property and the S-factor property.
Therefore, a short sampling time is preferred for storing an offset voltage of the driving transistor when the gray level is not influenced by the S-factor property, and a long sampling time is preferred for storing the offset voltage of the driving transistor T2 when the gray level is influenced by S-factor property. However, the sampling time in the related art OELD is fixed. Therefore, images of various gray levels are not displayed uniformly. In other words, an image of a gray level adequate for the fixed sampling time is displayed properly, but other images of gray levels inadequate for the fixed sampling time are not displayed properly. Therefore, in the related art OELD device, display uniformity is degraded.